EP (electrophotographic) process imaging devices, such as laser printers, typically use rotating polygon scanners to scan one or more focused laser spots across the width of a photosensitive element, such as a photosensitive drum or belt. FIG. 1 illustrates a conventional EP process device implemented as a laser printer 100. The plane swept out by a scanned laser beam 102(a) intersects a photosensitive element 104 in a scan line 106, and the passage of a focused laser spot along the scan line 106 exposes a row of pixels in the image being printed. Conventional imaging systems, such as that shown in laser printer 100, use pre-objective scanning, wherein a rotating polygon scanner 112 is located “before” an objective lens 108. More specifically, the “objective” 108 or scan lens 108 is located “downstream” from a rotating polygon scanner 112, between the polygon scanner 112 and the photosensitive element 104. The objective lens 108 in a pre-objective scanning system flattens the image field and forms well-focused image spots on the photosensitive element 104 over the full length of the scan line 106.
FIG. 2 illustrates a laser printer 200 in a post-objective scanning configuration, wherein the rotating polygon scanner 112 is located “after” the objective lens 202. More specifically, objective lens 202 is located “upstream” from the rotating polygon scanner 112, between the laser source 110 and the scanner 112. In a post-objective scanning system, the “upstream” objective lens 202 receives the laser beam as a stationary beam 102(b) rather than as the scanned laser beam 102(a) swept out by the rotating polygon scanner 112. Therefore, the objective lens 202 cannot correct the field curvature that is intrinsic to the polygon scanner 112 when the scanner is used with a converging input beam. Consequently, the image spots formed by the objective lens 202 are focused along a curved image surface 204 rather than along a straight scan line 106 as shown in FIG. 1. An image scanned across the photosensitive element 104 in such a post-objective scanning system will not be focused properly. Conventional imaging systems therefore use a pre-objective scanning configuration as shown in FIG. 1 in order to correct the image field curvature that would otherwise be created by a rotating polygon scanner 112.
Although conventional imaging systems using pre-objective scanning (e.g., FIG. 1) can effectively correct field curvature, such systems have various disadvantages. One disadvantage is the increased complexity of the design of the objective lens 108. An objective lens in a pre-objective scanning system must be able to tightly control X and Y spot position, field curvature, and wavefront quality across a wide image field. Polished glass and molded plastic elements used in such lenses typically have strongly aspheric surfaces, making the lenses challenging to design and difficult to manufacture. The cost of tooling needed to produce one of the large molded plastic lens elements typically exceeds one hundred thousand dollars. The large part size, long mold cycle time and tight tolerances contribute to a high per-part manufacturing cost. Pre-objective scan lens designs which satisfy all of the applicable performance requirements are highly constrained, limiting the lens designer's ability to extend existing lens designs to future printers having even wider fields and more scanning beams.
Another disadvantage of conventional imaging systems that use pre-objective scanning is an inability to compensate for system variations that can reduce print quality. As is well-known in the field of laser scanner design, the “focal plane” produced by the scan lens in a pre-objective scanning system generally consists of two independently curved focal surfaces, one for each of the two astigmatic focal directions. In many laser scanner designs, the uncorrected residual curvature of these focal surfaces is only slightly less than the optical system's allowable focus error (depth of focus), thereby reducing the system's tolerance to defocus due to other causes. For example, laser wavelength can vary for a particular laser diode (e.g., 110 of FIG. 1) as a function of applied power. Variations in laser wavelength may change the effective focal length of the optical system, causing a shift in the axial position of the focal surface and increasing the amount of residual field curvature that goes uncorrected by the objective lens. Time-dependent variations caused by thermal expansion or mechanical deformation of a print mechanism can also change image field curvature or focal surface position relative to the surface of the photosensitive element. Tolerances associated with replacement of the photosensitive element, especially when that element is contained within a replaceable cartridge or other subassembly, also contribute to focus error. Current imaging systems that use pre-objective scanning cannot compensate for such variations. All of these sources of focus error become more problematic in higher resolution scanning systems with increased numerical aperture and correspondingly reduced depth of focus.
Accordingly, the need exists for a way to correct field curvature and other focus errors in a laser imaging system that does not require the significant expense associated with the design and manufacture of objective lenses in current pre-objective scanning systems. The need also exists for a way to correct residual field curvature in a pre-objective scanner having an objective lens that only partially corrects field curvature. Such residual field curvature can exist, for example, when a simplified scan lens design is used or when the required performance level of the scanning system exceeds the capabilities of the best achievable scan lens design.